The goal of this procedure is to form strontium titanate bicrystals using a Spark Plasma Sintering apparatus in order to create grain boundary structures with well defined misorientations as a function of processing parameters, such as high pressure and heating rate. We need to understand whether grain boundaries generated during SPS are different from those produced during more traditional ceramic processing techniques. This will then help us generate new ceramic materials with unprecedented properties.
The main advantage of this technique is to form specific grain boundaries with well defined misorientations for the systematic understanding of microstructure formation during spark plasma sintering. To begin sample preparation, use a diamond wire saw to cut five millimeter by five millimeter pieces of a strontium titanate single crystal with a 1-0-0 plane surface polished to a mirror finish. Clean the samples with an ultrasonic cleaner in baths of acetone, isopropanol, and methanol for 15 minutes each at 50 to 60 Hertz.
Heat a hot plate to 200 degrees Celsius during the cleaning. Upon completing the ultrasonic cleaning in methanol, immediately place the samples on the hot plate to dry. Etch the samples in a six to one solution of ammonium fluoride and 49%hydrofluoric acid for 10 minutes.
Rinse the etched samples in deionized water and isopropanol in sequence. Dry the samples with filtered dry air. Discard any samples that developed a rainbow patina on the optically flat side.
First, place a 30 millimeter circle of graphite paper on a 30 millimeter diameter graphite plunger. Then, stack two STO single crystal samples with their optically flat sides facing each other on the center of the paper covered plunger. Rotate the top single crystal around the 1-0-0 axis to the desired misorientation angle.
Carefully slide a 30 millimeter diameter graphite die over the crystals and plunger. Insert another 30 millimeter circle of graphite paper and plunger into the die on top of the crystal stack. Place the assembly on the lower graphite spacer of the SPS apparatus.
And then place the other graphite spacer on top of the assembly. Use the z-axis control buttons to bring the upper punch down to apply a three kilonewton uniaxial force to the assembly. Insert a K type thermocouple into the side of the graphite die to reach the sample.
Set the chamber pressure to approximately 10 Pascal. Then on the instrument program control, set the bond temperature, time, and heating rates for the experiment. Select sinter conditions of a DC pulse of 12 seconds on, two seconds off, and then start the sintering program.
Once the program ends, allow the sample to cool to 150 degrees Celsius. Transfer the gray black reduced sample to a high temperature furnace. Anneal the sample at 1200 degrees Celsius for 140 hours in air at ambient pressure to obtain the off-white oxidized bicrystal.
Use a diamond wire saw to slice the bicrystal into five millimeter by one millimeter pieces. Polish the cross-sections with decreasing grit sizes of diamond lapping film until the scratches are uniform at the smallest grit size. Then start a two minute polish of the cross-sections with continuously poured colloidal silica and a matte cloth.
15 seconds before polishing ends, switch from pouring colloidal silica to pouring DI water onto the platen. Immediately rinse the polish sample in DI water for one minute. Clean the polished sample in consecutive ultrasonic cleaning baths of acetone, isopropanol, and methanol for 15 minutes each.
Immediately after cleaning, dry the sample on a hot plate at 200 degrees Celsius. Melt the clean sample polished side up on a sample stub coated with colloidal graphite. Sputter coat the sample surface with two to three nanometers of carbon.
First, place the focused ion beam copper grid in consecutive ultrasonic cleaner baths of acetone and isopropanol for one hour. Then, plasma clean the copper grid for 10 minutes. Place the STO bicrystal sample and the clean copper grid in the FIB apparatus with the sample stage set at seven millimeters.
In the FIB instrument software, identify a region of interest along the grain boundary. Navigate to patterning control and select the rectangle patterning tool with the CE depth surface application for electron beam deposition of carbon. Insert the carbon gas injection needle and deposit a 15 by two by two micrometers cubed protective layer of carbon over the region of interest.
Then, retract the injection needle. Return to patterning control and choose the rectangle patterning tool with a W dep application for ion beam deposition of tungsten. Insert the tungsten gas injection needle and deposit a protective layer of tungsten of the same dimensions over the region of interest.
Retract the injection needle. In patterning control, select the regular cross section patterning tool with the silicon application property. Mill a 22 by 27 by 15 micrometer cubed trench above the protected ROI and a 22 by 25 by 15 micrometer cubed trench below the ROI.
Then, in patterning control, use the silicon clean cross section patterning tool to clean the trenched lamella surface. Next, use the silicon rectangle patterning tool to cut a two micrometer wide J pattern with two micrometer wide rectangles into the sample. Switch to the easy lift dialog box and set the micromanipulator to the park position.
Lower the micromanipulator and weld the sample to the tip of the micromanipulator by tungsten deposition. Then, extend the J patten into a U pattern. Lift the lamella from the bulk sample and rotate the micromanipulator 180 degrees to set the grain boundary perpendicular to the ion beam.
Tungsten welds with a lamella to the copper grid. Then, cut the micromanipulator free from the lamella using the rectangle patterning tool. Deposit a 15 by two by four micrometer cubed layer of carbon and a 15 by two by eight micrometer cubed layer of tungsten over the area of interest.
Then, with the ion beam, thin the sample first to approximately 200 nanometers with a cleaning cross section pattern. And then, with a rectangle pattern, thin the sample to electron transparency. Remove the copper FIB grid from the FIB apparatus and place the grid onto a NanoMil sample holder.
Then place the sample holder into the NanoMil apparatus. Using the NanoMil apparatus software, mil the sample at 500 electron volts for 10 minutes to remove amorphous damage caused by the FIB. Place the cleaned TEM sample in a sample holder and insert the sample and holder into the STEM.
In the STEM software, use the capture tool to obtain an image of the sample grain boundary. Strontium titanate bicrystals were formed from single crystal pairs at various misorientation angles using a Spark Plasma Sintering apparatus. The interface was considered to be bonded wherever the grain boundary was not observable in SEM imaging.
Areas of dark contrast in the images are either attributed to voids or colloidal graphite diffusion between the single crystals. The maximum bonded interface fraction was achieved with zero degrees of misorientation, a bonding temperature of 600 or 700 degrees Celsius, a bonding duration of 90 minutes, and an annealing time of 100 hours. Increasing the angle of misorientation to approximately 45 degrees necessitated a higher bonding temperature and a longer annealing time to achieve a 45%bonded interface.
High resolution TEM and high angle annular dark field STEM show an atomically abrupt grain boundary with no observed second phases or intergranular film. Electron energy loss spectroscopy at the interface showed decreased crystal field splitting of the titanium L2 and L3 edges and decreased intensity in the oxygen K edge compared to the bulk crystal, suggesting a greater concentration of oxygen vacancies at the interface. Following the same procedure, we can also form doped grain boundaries using the SPS.
This will help us to address whether the electric field will change grain boundary segregation. After watching this video, you should have a good understanding of how to form bicrystals with atomically abrupt interface structures using SPS. Don't forget that hydrofluoric acid can be extremely hazardous and precautions, such as personal protective wear, should be taken.
